Filing joint in 2018, I deducted all the mortgage interest on a $800k home loan. In 2019, I am widowed, will I still be able to deduct all the mortgage interest?

I am sorry for your loss.

Yes - You should be able to deduct all the mortgage interest on your return.

[NOTE:  You will still file a joint return with your spouse for the year that they died.  If your spouse died in 2019, you would still include them and file jointly on your 2019 return.]